The relay valve of modern tractor-trailer combinations constitutes an important part of the air brake system. It is connected to the control line from the operator, to a reservoir constituting the source of operating pressure for the service brake, and to the service brake as well as providing discharge of released brake pressure to atmosphere. The operating response, the reliability and the pressure drop through the relay valve are all of critical importance to the brake system as a whole.
In the past, many types of relay valves have been developed, each with different criteria in mind. Attempts have been made to achieve a pressure balanced relay valve through pressure equalization features such as the use of a single "O" ring to seal the housing to both a piston and a shuttle as disclosed in U.S. Pat. No. 4,096,884 to C. Horowitz issued June 27, 1978. Pressure balancing has also been an objective of the design disclosed in U.S. Pat. No. 3,858,610 to B. Klimek issued Jan. 7, 1975 and U.S. Pat. No. 3,512,552 to Dobrikin et al issued May 19, 1970. Proportioning relay valves have been disclosed in U.S. Pat. Nos. 3,944,294 to Masuda et al, issued Mar. 16, 1976, 3,936,097 issued to Yanagawa et al on Feb. 3, 1976, 3,945,689 to Masuda et al issued Mar. 23, 1976. A combined load sensing and relay valve is disclosed in U.S. Pat. No. 4,045,094 issued to Yanagawa et al on Aug. 30, 1977.
Despite the state of development of relay valves, none has achieved a true low opening pressure, fast response time and a pressure balance across both the valve piston and spool or shuttle in one valve. In fact, none, to our knowledge has achieved low operating pressures in the order of one or two psi at the control line input. None has achieved a basic relay valve design which may be converted to a booster valve by the simple addition of an intermediary plate.